This invention relates to a fluidized bed combustor and, more particularly, to such a combustor having two parallel stages for generating gases to drive a turbine.
Combustion systems utilizing fluidized beds as the primary source of heat generation are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulphur generated by the combustion of the coal, to fluidize the bed and promote the combustion of the fuel at relatively low temperatures. When the combustor is pressurized, the hot gases produced by the fluidized bed can be used to drive a turbine for the generation of electrical power.
One of the characteristics of a fluidized bed combustor is a relatively low gas temperature which is necessary in order to keep the pollutant emissions low. However, this results in a compromise in the combined boiler-turbine cycle efficiency since the turbine can be operated with gas temperatures well in excess of the maximum permitted in a fluidized bed boiler. Hence, in order to increase the temperature of the gas entering the turbine to improve the cycle efficiency, it has been proposed to gasify a slip stream of the fuel feed in a separate vessel. This fuel gas stream then combines with the flue gases from the boiler in a burner to raise the temperature of the latter gases to acceptable levels. However, this requirement of an additional gasifier considerably adds to the cost of the process and is thus undesirable.